Mixture of pure oils can be unsafe (8)
I believe the answer is:
perilous
'unsafe' is the definition.
('perilous' can be similar in meaning to 'unsafe')
'mixture of pure oils' is the wordplay.
'mixture of' indicates an anagram (I've seen this in other clues).
'pure'+'oils'='pureoils'
'pureoils' is an anagram of 'PERILOUS'.
'can be' is the link.
(Other definitions for perilous that I've seen before include "Fraught with danger" , "Hazardous, dangerous" , "Risky" , "Extremely dangerous" , "Hazardous, menacing" .)